Knowledge of the structure about the metal ion in the active site of metalloenzymes is crucial for the understanding and interpretation of the enzyme-substrate reaction mechanism. We propose to investigate the structure of metalloenzymes in the vicinity of the metal ion during substrate reaction and inhibition using novel techniques which focus on the hyperfine interactions of the nucleus of the metal ion complexed by the enzyme. We wish to study the strength, symmetry and time dependence of the ligand fields at the metal ion using 1) The perturbed angular correlation (PAC) of nuclear gamma-rays emitted by a suitable radioactive tracer (eg. Co57) substituted at the active site, and 2) Electron paramagnetic resonance (EPR) studies of enzyme single crystals at 4.2K. The substitution of Co57(II) for the normal Zn(II) ion in the three enzymes - carbonic anhydrase, carboxypeptidase, and alkaline phosphatase - chosen for study still leave the enzyme active. Information thus obtained in these experiments about the static and time-dependent molecular environment of the cobalt ion can be used to help characterize further the detailed mechanism of enzymatic activity.